Electrically conductive adhesive

ABSTRACT

Disclosed herein is an electrically conductive adhesive composition, which including milled carbon fibers dispersed in a thermosetting resin and a curative agent. Also, disclosed herein are articles comprising at least two components adhesively bonded by the electrically conductive adhesive composition and methods of making such adhesives and articles.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.17/041,735, filed Sep. 25, 2020, which is a national stage applicationunder 35 U.S.C. § 371 of International Application No.PCT/US2019/024563, filed Mar. 28, 2019, which is, related to, and claimsthe benefit of priority of U.S. Provisional Application No. 62/649,054,entitled ELECTRICALLY CONDUCTIVE ADHESIVE, filed on Mar. 28, 2018, thecontents of each of these applications are incorporated herein byreference in their entireties for all purposes.

FIELD OF THE INVENTION

The present disclosure relates generally to electrically conductiveadhesive composition including milled carbon fibers dispersed in athermosetting resin and a curative agent. The present disclosure alsorelates generally to articles comprising at least two componentsadhesively bonded by the electrically conductive adhesive compositionand methods of making such adhesives and articles.

BACKGROUND OF THE INVENTION

Many electrically conductive surfaces when bonded using an adhesivedisplay good longitudinal or surface electrical conductivity, but poorthrough-thickness transverse electrical conductivity. Hence, there is aneed for an improved electrically conductive adhesive for bondingelectrically conductive surfaces and for improving the through-thicknesstransverse electrical conductivity of bonded surfaces.

SUMMARY OF THE INVENTION

In an aspect, there is an electrically conductive adhesive compositioncomprising milled carbon fibers dispersed in a thermosetting resin and acurative agent.

In another aspect, there is an article comprising at least twocomponents, where a portion of a first component is adhesively bonded toa portion of a second component using the electrically conductiveadhesive composition of the present invention.

In yet another aspect, there is a spar cap comprising a plurality offiber-reinforced resin composite sheets adhesively bonded to each otherusing the electrically conductive adhesive composition of the presentinvention, wherein each fiber-reinforced resin composite sheet comprises50 to 80% by volume of carbon fibers fused with a binder resin, whereinthe amount in % by volume is based on the total volume of thefiber-reinforced resin composite sheet.

In yet another aspect, there is a method comprising the steps ofdispersing milled carbon fibers in a thermosetting resin to form amixture and adding a curative agent to the mixture and optionally curingto form an electrically conductive adhesive.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate one (several) embodiment(s) ofthe invention, and together with the written description, serve toexplain certain principles of the invention.

FIG. 1 shows a method of making an electrically conductive adhesive, inaccordance with various embodiments of the present invention.

FIG. 2 shows another method of making an electrically conductiveadhesive, in accordance with various embodiments of the presentinvention.

FIG. 3 shows results of difference in the mixing methods: without andwith vacuum.

FIG. 4 shows a method of application of electrically conductive adhesiveof the present invention on to a surface of a panel.

FIG. 5 shows a sectional view of a portion of an exemplary articlecomprising a composite panel, in accordance with various embodiments ofthe present invention.

FIG. 6 shows a sectional view of a portion of another exemplary articlecomprising a composite panel, in accordance with various embodiments ofthe present invention.

FIG. 7 shows an exemplary schematic sectional diagram of a portion of aspar cap, in accordance with various embodiments of the presentinvention.

FIG. 8 shows another exemplary schematic sectional diagram of a portionof a spar cap, in accordance with various embodiments of the presentinvention.

FIG. 9 shows an exemplary experimental set-up for measuring throughthickness electrical conductivity in accordance to ANSI ANSI/ESDSTM11.12.

FIG. 10 shows a graph showing through-thickness electrical conductivityof a composite panel comprising two pultruded sheets adhesively bondedby the electrically conductive adhesive of the present invention made bytwo different mixing methods, as a function of filler content, whereineach pultruded sheet comprises 20-80% by volume of carbon fiber fusedwith a vinyl ester resin.

FIG. 11 shows a graph showing through-thickness electrical conductivityof a composite panel comprising two pultruded sheets adhesively bondedby the electrically conductive adhesive of the present invention, as afunction of filler content, wherein each pultruded sheet comprises20-80% by volume of carbon fiber.

FIG. 12 shows a graph showing through-thickness electrical conductivityof a composite panel comprising two pultruded sheets adhesively bondedby the electrically conductive adhesive of the present invention made bytwo different mixing methods, as a function of filler content, whereineach pultruded sheet comprises 20-80% by volume of carbon fiber fusedwith an epoxy resin.

FIG. 13 shows the results of an electrical strike in a non-conductiveadhesive joint and a conductive adhesive joint.

FIG. 14 shows a picture of an exemplary C-section, in accordance withvarious embodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Although the invention is illustrated and described herein withreference to specific embodiments, the invention is not intended to belimited to the details shown. Rather, various modifications may be madein the details within the scope and range of equivalents of the claimsand without departing from the invention.

The wind energy industry requires lightning protection for the blades inwhich carbon fiber-reinforced resin composites have been commonly usedfor the spar caps Procured carbon fiber-reinforced resin composites aretypically bonded together in a stack using a binder resin which is notelectrically conductive. Due to the strong anisotropy of the electricalconductivity of the carbon fibers and uni-directional planar tow form ofthese carbon fiber-reinforced resin composites, the lengthwiseconductivity of these carbon fiber-reinforced resin composites can befour orders of magnitude higher than transverse conductivity. Hence, asa result of this anisotropy, during a lightning strike, the high energyelectricity would flow lengthwise, but would arc transversely, whichcould result in elevation in temperature, and thereby can causedelamination or incineration of the spar cap. Therefore, it has beendiscovered that there is a need to effectively enhance thethrough-thickness transverse electrical conductivity of the carbonfiber-reinforced resin composites forming the spar cap. It has beencontemplated to increase the electrical conductivity of materials byvarious techniques, such as:

-   -   Incorporating nanomaterials in adhesives. However, nanomaterials        are expensive and may require special precautions in handling        further increasing COM, as powdered nanomaterials can be        considered a hazardous material in some environments.    -   Incorporating metal particles and/or metal coated particles in        adhesives.    -   Improving conductivities of prepreg materials rather than        adhesives    -   Some targeted film making    -   Some targeted surface films and treatments

To the extent that these approaches have various issues, alternativeapproaches may be more efficient and cost effective for enhancing theelectrical conductivity of adhesives. Further, enhancing the electricalconductivity of adhesives provides a way to assure electrical continuitythrough the entire thickness of a bonded stack of plates in a spar cap.It is the objective of the present invention to effectively enhance theelectrical conductivity of adhesives.

Disclosed herein is an electrically conductive adhesive composition,articles comprising at least two components adhesively bonded by theelectrically conductive adhesive composition and methods of makingthereof.

In an aspect, there is an electrically conductive adhesive compositioncomprising milled carbon fibers dispersed in a thermosetting resin and acurative agent. The milled carbon fibers can be present in an amount inthe range of 2 to 40 weight %, or 1 to 50 weight %, based on the totalweight of the electrically conductive adhesive composition.

In an embodiment, the adhesive composition has a linear conductivity inthe range of 10⁻¹² to 10 ⁻⁸ Siemens/m (S/m).

Any suitable milled carbon fibers can be used, whether they are milledfrom pristine carbon fibers or recycled carbon fibers. In an embodiment,the milled carbon fibers are recycled milled carbon fibers, free ofsizing. As used herein, the phrase “recycled milled carbon fibers, freeof sizing” refers to carbon fibers that end up as waste at some point intheir lifecycle. The fiber source of the recycled milled carbon fiberscould be fiber that was deemed unsuitable for continuous or choppedfiber product, fiber that was cut off during normal production of thefiber, or left over from the manufacturing process of carbon fiberintermediate products. Additionally, there is a little bit of fiber leftover at the end of each fiber production run—that can be used as fibersource for making recycled milled carbon fiber. These wasted carbonfibers are stripped of any sizing that they might have had before beingmilled and converted into milled carbon fibers.

In an embodiment, the milled carbon fibers have an electricalresistivity of about 0.00155 Ohm-cm. The milled carbon fibers can havean average diameter in the range of 5 to 10 microns, and an averagelength in the range of 5 to 300 microns. In an embodiment, milled carbonfibers can have an average diameter of 7.2 microns and an average lengthof 100 microns. Suitable examples of commercially available milledcarbon fibers include, but are not limited to: PX35MF0150, PX35MF0200,all available from Zoltek Corporation.

Any suitable thermosetting resin can be used, including, but not limitedto at least one of epoxy resin, vinyl ester resins, phenolic resins(such as, phenolic formaldehyde (PF) resins, phenolic neoprene,resorcinol formaldehydes (RF)), unsaturated polyesters, thermosetpolyurethanes, polyimides, bismaleimide resins, benzoxazine resins,silicone resins, and any combination, copolymer, and/or derivativethereof.

In an embodiment, the thermosetting resin is an epoxy resin. Someexamples of commercially available thermosetting resin include, but arenot limited to: Sikadur WTG-1280 Part A, available from SikaCorporation; Körapur 4W-Komp A, available from Kömmerling Corporation.

In another embodiment, the curative agent comprises an amine functionalgroup. Some examples of commercially available curative agent include,but are not limited to: Sikadur WTG-1050 Part B, available from SikaCorporation; and Köracur 4W-Komp B available from KömmerlingCorporation.

In yet another embodiment, the thermosetting resin and the curativeagent are available as a two component adhesive kit. Some examples ofcommercially available two component adhesive kit include, but are notlimited to: Sikadur WTG-1280, available from Sika Corporation; andKörapur 4W available from Kömmerling Corporation.

In an embodiment, the electrically conductive adhesive composition ofthe present invention is suitable for use in the wind blade bondingapplications.

The electrically conductive adhesive of the present disclosure could beused for various conductive bonding applications besides spar caps, suchas the connection of the cables connecting the spar cap to the main downconductor cable that is part of the lightning protection system for theblade.

Method of Making an Adhesive Composition

In an aspect, there is a method comprising the steps of dispersingmilled carbon fibers in a thermosetting resin to form a mixture, addinga curative agent to the mixture, followed by mixing thoroughly andoptionally curing to form an electrically conductive adhesive. Anysuitable mechanical means can be used for the step of dispersing milledcarbon fibers in a thermosetting resin to form a mixture. In anembodiment, the step of adding curative agent to the mixture and mixingis done in air, or under inert environment. In another embodiment, thestep of adding curative agent to the mixture and mixing is done undervacuum. The amount of time required for each mixing step will dependupon the composition and the mechanical means and can be in the range 30seconds to 60 minutes.

Article

In another aspect, there is an article comprising at least twocomponents, where a portion of a first component is adhesively bonded toa portion of a second component using the electrically conductiveadhesive composition, as disclosed hereinabove.

Any suitable material can be used for the components. In one embodiment,at least one of the at least two components comprises a fiber-reinforcedresin composite. The fiber-reinforced resin composite may include afiber and a binder resin.

In an embodiment of the fiber-reinforced resin composite, the fiber is acarbon fiber. In another embodiment, the fiber is an electricallyconductive sized fiber.

In yet another embodiment, the fiber-reinforced resin compositecomprises carbon fibers fused with a binder resin.

The fiber-reinforced resin composites according to the present inventionmay be formed from and based on any binder resin known in the art.

Non-limiting examples of a binder resin that is a thermoset (co)polymerincludes unsaturated polyesters, epoxy resins, vinyl ester resins,phenolic resins, thermoset polyurethanes, polyimides, bismaleimideresins, benzoxazine resins, and silicone resins.

Non limiting examples of a binder resin that is a thermoplastic(co)polymer includes, polyolefins, cyclic polyolefins, acrylonitrilebutadiene styrene, polyvinyl chloride, polystyrene, thermoplasticpolyesters, polyvinyl alcohols, polymethyl methacrylates, styrene maleicanhydrides, polyoxymethylene (acetals), thermoplastic polyurethanes,polyethylene terephthalates, polytrimethylene terephthalates,polybutylene terephthalates, polyamides, polycarbonates,polyvinylpyrrolidone, polytetrafluoroethylene, polysulfones,polyethersulfones, polyphenylene sulfides, polyetherimides,polyamide-imides, polyetheretherketones, and polyaryletherketones,including alloys and blends.

In an embodiment, the fiber-reinforced resin composite is in the form ofa pultruded sheet, a fabric, or a prepreg. In another embodiment, thefiber-reinforced resin composite in the form of a pultruded sheetincudes carbon fibers in a planar tow form fused with the binder resin.In yet another embodiment, the fiber-reinforced resin composite in theform of a fabric, with the fiber being a multidirectional fabric, aunidirectional fabric or a woven fabric.

In an embodiment, the fiber-reinforced resin composite includes at leastone of 20-80% by volume of fiber reinforcement and a vinyl ester resin,20-80% by volume of fiber reinforcement and a polyester resin or 20-80%by volume of fiber reinforcement and an epoxy resin, where the amount in% by volume is based on the total volume of the fiber-reinforced resincomposite.

In an embodiment, the fiber-reinforced resin composite is in the form ofa pultruded sheet, a resin-infused fabric, a pre-impregnated tape, or asheet molding compound. In the pultruded form, the fiber may be presentin an amount in the range of 40-80%, or preferably 60-72%, by volume,based on the total volume of the fiber-reinforced resin composite. Inthe pultruded form, the fiber may be present in an amount in the rangeof 40-80%, or preferably 60-72%, by volume, based on the total volume ofthe fiber-reinforced resin composite. In the prepreg form, the fiber maybe present in an amount in the range of 40-80%, or preferably 54-61%, byvolume, based on the total volume of the fiber-reinforced resincomposite. In the resin-infused fabric form, the fiber may be present inan amount in the range of 40-80%, or preferably 48-59%, by volume, basedon the total volume of the fiber-reinforced resin composite.

The fiber-reinforced resin composite may also include any filler and/orparticle, known in the art for reinforcing composites, such as polymercomposites. Examples of such particles include, but are not limited to,talc, calcium carbonate, aluminum hydroxide, titanium oxides, andsilica.

Article

In an aspect, there is an article comprising at least two componentsadhesively bonded to each other using the electrically conductiveadhesive composition, where at least one of the at least two componentsincludes the fiber-reinforced resin composite, as disclosed hereinabove.

FIG. 5 shows a sectional view of a portion of an exemplary articlecomprising a composite panel, in accordance with various embodiments ofthe present invention. The exemplary composite panel includes twocomponents, a first component adhesively bonded to a second componentusing the electrically conductive adhesive composition disposed betweenand in contact with the first component and the second component. In anembodiment, at least one of the two components comprises afiber-reinforced resin composite.

FIG. 6 shows a sectional view of a portion of another exemplary articlecomprising a composite panel comprising a plurality of panels adhesivelybonded to each other using the electrically conductive adhesivecomposition of the present disclosure. As shown in FIG. 6 , theexemplary composite panel comprises at least four panels, a first panelis adhesively bonded to a second panel, the second panel adhesivelybonded to a third panel and the third panel adhesively bonded to afourth panel using the electrically conductive adhesive compositiondisposed between and in contact with the each panel. In an embodiment,the panels are stacked on top of each other such that at least one edgeis slanted and has a slope. In an embodiment, at least one of theplurality of panels comprises a fiber-reinforced resin composite.

The composite panels of the present invention can have any suitablethickness, such as in the range of 0.5-30 mm.

In an embodiment, the article is a spar cap including a plurality offiber-reinforced resin composite sheets adhesively bonded to each otherusing the electrically conductive adhesive composition of the presentinvention. FIG. 10 shows a schematic sectional diagram of a portion of aspar cap.

FIG. 7 shows an exemplary schematic sectional diagram of a portion of aspar cap, in accordance with various embodiments of the presentinvention. In particular, FIG. 7 shows a configuration of bonded platesin a spar cap.

FIG. 8 shows another exemplary schematic sectional diagram of a portionof a spar cap, in accordance with various embodiments of the presentinvention. In particular, FIG. 8 shows the use of the conductiveadhesive to cover the end of the chamfer region of one plate and thesurrounding surface area of the chamfered plate and the adjoining plate.

In an embodiment of the spar cap, the fiber-reinforced resin compositesheet comprises 20-80% by volume of carbon fibers fused with a vinylester resin. In another embodiment of the spar cap, the fiber-reinforcedresin composite sheet comprises 20-80% by volume of fused with an epoxyresin. In another embodiment of the spar cap, the fiber-reinforced resincomposite sheet comprises 20-80% by volume of fused with a polyesterresin. In yet another embodiment of the spar cap, at least one of theplurality of fiber-reinforced resin composite sheets is a pultrudedsheet comprising 40-80%, or preferably 60-72%, by volume of fiber, basedon the total volume of the fiber-reinforced resin composite.

In an embodiment, there is an article comprising at least twocomponents, where a portion of a first component is adhesively bonded toa portion of a second component by an electrically conductive adhesiveof the present disclosure. In an embodiment, the article is a spar capand the two components form a chamfer region. In another embodiment, thearticle further comprises the electrically conductive adhesive disposedover at least one of the at least two components. In yet anotherembodiment, the electrically conductive adhesive extends beyond thechamfer region onto both surfaces of the adjoining components.

It is believed that the electrical conductivity would be maintainedbetween the adhesively bonded fiber-reinforced resin composite sheetsusing the conductive adhesive composition of the present application,thereby minimizing the arcing phenomenon. The resulting spar cap of thepresent invention will be far more lightning resistant that one withouta conductive interlayer between the fiber-reinforced resin compositesheets.

Method of Making a Composite Panel

In an aspect, there is a method comprising applying the electricallyconductive adhesive on a portion of a surface of a first panel; andadhesively bonding a second panel to the first panel, such that theelectrically conductive adhesive is disposed between the first and thepanel to form a composite panel. Any suitable method can be used for theapplication of adhesive, including but not limited to spray, webcoating, wheel or brush coating. The method of application is based onspecific composition of the electrically conductive adhesive, the natureof the panels and production requirements. The method can furtherinclude curing the composite panel. The curing can be done thermally orphotochemically, depending upon the nature of the curative agent presentin the adhesive composition. The curing can be done in air, under inertenvironment or under vacuum.

In an embodiment, at least one of the first or the second panelcomprises a fiber-reinforced resin composite, as disclosed hereinabove.

In an aspect, the method may further include forming a fiber-reinforcedresin composite in the form of a pultruded sheet or a resin-infusedfabric, or a pre-impregnated tape (prepreg, impregnated with athermoplastic resin or a thermosetting resin), or a sheet moldingcompound (SMC). In an embodiment, the step of forming a fiber-reinforcedresin composite can include arranging a fiber into a fabric and infusingthe fabric with a binder resin to form a resin infused fabric or aprepreg. In another embodiment, the step of forming a fiber-reinforcedresin composite can include arranging a fiber into a planar tow form,infusing the planar tow form with a binder resin and pultruding theresin infused planar tow form to form a pultruded sheet.

In another embodiment, the step of forming a fiber-reinforced resincomposite can include arranging a combination of a fiber and a fabricinto a structural cross section form, infusing the structural crosssection form with a binder resin and pultruding the resin infusedsection form to result in a pultruded structural section (for example aC-section, J-section, or Pi-section). FIG. 14 shows a picture of anexemplary C-section, in accordance with various embodiments of thepresent invention.

In another embodiment, the step of forming a fiber-reinforced resincomposite can include compounding at least one of a plurality of choppedor a plurality of continuous fibers with a binder resin and compressionmolding or injection molding the resulting composition into an article.

In another embodiment, the method may include a step of making acomposite panel in the form of a spar cap.

In an embodiment, the electrically conductive adhesive composition ofthe present invention is suitable for use in the wind bladeapplications.

The use of the electrically conductive adhesive composition of thepresent invention in adhesively bonding at least two components,provides several advantages over conventional adhesives, including, butnot limited to:

-   -   Low cost, as the milled fiber is much less expensive than the        nanomaterials, as disclosed in the prior art. The characteristic        dimension of nanoparticles is not needed for this application        and hence would be an unnecessary cost burden. Furthermore, no        special precautions must be taken with the powdered form of        milled carbon fiber. In contrast, powdered nanomaterials can be        considered a hazardous material in some environments.    -   Due to their aspect ratio, the milled fibers impart some        structural/mechanical benefit to the adhesive.    -   Milled carbon fiber assures perfect compatibility with the        carbon plates being bonded.    -   Relatively low levels of milled carbon fiber are needed to        assure electrical continuity through the thickness of a stack of        carbon plates, as shown in FIGS. 7-9 .    -   For the 2-part adhesive, the milled fiber can be premixed into        the Part A (resin component), thereby providing to the user a 2        part electrically conductive adhesive. Hence, the user will        maintain a common process when mixing and using the 2 part        electrically conductive adhesive. Additionally, this enables        point-of-use mixing with automatic mixing tube applicators.    -   For the wind blade manufacturing application, using the two-part        adhesives already used provides maintenance of the process        familiarity, mechanical bonding properties, and handling        characteristics.    -   In contrast to all other known approaches, the present invention        discloses the use of milled carbon fiber as the conductive        filler. By using the resulting conductive adhesive in the        lamination of procured carbon composites, the electrical        conductivity reduces the flash over of electricity between        layers during a lightning strike event in wind turbine blades.

More specifically, the following represents particular embodiments ofthe invention:

-   -   1. An electrically conductive adhesive composition comprising:        -   a) milled carbon fibers dispersed in a thermosetting resin;            and        -   b) a curative agent.    -   2. The electrically conductive adhesive composition according to        embodiment 1, wherein the thermosetting resin is an epoxide        resin.    -   3. The electrically conductive adhesive composition according to        embodiments 1 or 2, wherein the curative agent comprises an        amine functional group.    -   4. The electrically conductive adhesive composition according to        any one of the preceding embodiments, wherein the thermosetting        resin comprises at least one of epoxy resin, vinyl ester resins,        phenolic resins, unsaturated polyesters, thermoset        polyurethanes, polyimides, bismaleimide resins, benzoxazine        resins, silicone resins, and any combination, copolymer, and/or        derivative thereof.    -   5. The electrically conductive adhesive composition according to        any one of the preceding embodiments, wherein the milled carbon        fibers are present in an amount in the range of 1 to 20 weight        %, based on the total weight of the electrically conductive        adhesive composition    -   6. The electrically conductive adhesive composition according to        any one of the preceding embodiments, wherein the milled carbon        fibers have an average diameter in the range of 5 to 10 microns        and an average length in the range of 1 to 300 microns.    -   7. The electrically conductive adhesive composition according to        any one of the preceding embodiments, wherein the adhesive        composition has a linear conductivity in the range of 10⁻¹²10⁻⁸        Siemens/m (S/m).    -   8. The electrically conductive adhesive composition of according        to any one of the preceding embodiments, wherein the milled        carbon fibers are free of sizing.    -   9. The electrically conductive adhesive composition of according        to any one of the preceding embodiments, wherein the milled        carbon fibers are pristine or recycled milled carbon fibers,        free of sizing.    -   10. An article comprising at least two components, where a        portion of a first component is adhesively bonded to a portion        of a second component using the electrically conductive adhesive        composition according to any one of the embodiments 1-9.    -   11. The article according to embodiment 10, wherein at least one        of the at least two components comprises a fiber-reinforced        resin composite.    -   12. The article according to embodiment 11, wherein the        fiber-reinforced resin composite comprises carbon fibers fused        with a binder resin.    -   13. The article according to embodiment 10, wherein at least one        of the at least two components is in the form of a pultruded        sheet, a resin-infused fabric, a pre-impregnated tape, or a        sheet molding compound.    -   14. The article according to embodiment 12, wherein the        pultruded sheet comprises carbon fibers in a planar tow form        fused with a binder resin.    -   15. The article according to embodiment 12, wherein the binder        resin comprises a thermosetting binder resin chosen from among        unsaturated polyesters, epoxy resins, vinyl ester resins,        phenolic resins, thermoset polyurethanes, polyimides,        bismaleimide resins, benzoxazine resins, and silicone resins.    -   16. The article according to embodiment 12, wherein the binder        resin comprises a thermoplastic binder resin chosen from among        polyolefins, cyclic polyolefins, acrylonitrile butadiene        styrene, polyvinyl chloride, polystyrene, thermoplastic        polyesters, polyvinyl alcohols, polymethyl methacrylates,        styrene maleic anhydrides, polyoxymethylene (acetals),        thermoplastic polyurethanes, polyethylene terephthalates,        polytrimethylene terephthalates, polybutylene terephthalates,        polyamides, polycarbonates, polyvinylpyrrolidone,        polytetrafluoroethylene, polysulfones, polyethersulfones,        polyphenylene sulfides, polyetherimides, polyamide-imides,        polyetheretherketones, and polyaryletherketones, including        alloys and blends.    -   17. The article according to any one of embodiments 13, wherein        the fabric comprises a multidirectional fabric, a unidirectional        fabric or a woven fabric.    -   18. The article according to embodiment 13, wherein the        pultruded sheet comprises at least one of:        -   a) 20-80% by volume of carbon fibers fused with a vinyl            ester resin, or        -   b) 20-80% by volume of carbon fibers fused with an epoxy            resin,        -   wherein the amount in % by volume is based on the total            volume of the fiber-reinforced resin composite.    -   19. A spar cap comprising a plurality of fiber-reinforced resin        composite sheets adhesively bonded to each other using the        adhesive composition according to any one of the embodiments        1-9,        -   wherein each fiber-reinforced resin composite sheet            comprises 50 to 80% by volume of carbon fibers fused with a            binder resin, wherein the amount in % by volume is based on            the total volume of the fiber-reinforced resin composite            sheet.    -   20. The spar cap according to embodiment 19, wherein the        fiber-reinforced resin composite sheet comprises 20-80% by        volume of carbon fibers fused with a vinyl ester resin.    -   21. The spar cap according to embodiment 19, wherein the        fiber-reinforced resin composite sheet comprises 20-80% by        volume of carbon fibers fused with an epoxy resin.    -   22. A method comprising:        -   a) dispersing milled carbon fibers in a thermosetting resin            to form a mixture; and        -   b) adding a curative agent to the mixture and optionally            curing to form an electrically conductive adhesive.    -   23. The method according to embodiment 22 wherein the step of        dispersing milled carbon fibers in a thermosetting resin to form        a mixture is done mechanically.    -   24. The method according to embodiment 23, wherein the step of        adding a curative agent to the mixture further comprises mixing        under vacuum.    -   25. The method according to according to any one of the        embodiments 22-24, further comprising:        -   a) applying the electrically conductive adhesive on a            portion of a surface of a first component; and        -   b) adhesively bonding a second component to the first            component, such that the electrically conductive adhesive is            disposed between the first and the components to form a            composite panel.    -   26. The method according to embodiment 25, wherein at least one        of the first or the second components comprises a        fiber-reinforced resin composite.    -   27. The method according to embodiment 26, wherein the        fiber-reinforced resin composite comprises carbon fibers fused        with a binder resin.    -   28. The method according to embodiment 26, wherein the        fiber-reinforced resin composite is in the form of a pultruded        sheet, a resin-infused fabric, a pre-impregnated tape, or a        sheet molding compound.    -   29. The method according to embodiment 28, wherein the pultruded        sheet comprises carbon fibers in a planar tow form fused with a        binder resin.    -   30. The method according to embodiment 27, wherein the binder        resin comprises a thermosetting binder resin chosen from among        unsaturated polyesters, epoxy resins, vinyl ester resins,        phenolic resins, thermoset polyurethanes, polyimides,        bismaleimide resins, benzoxazine resins, and silicone resins.    -   31. The method according to embodiment 27, wherein the binder        resin comprises a thermoplastic binder resin chosen from among        polyolefins, cyclic polyolefins, acrylonitrile butadiene        styrene, polyvinyl chloride, polystyrene, thermoplastic        polyesters, polyvinyl alcohols, polymethyl methacrylates,        styrene maleic anhydrides, polyoxymethylene (acetals),        thermoplastic polyurethanes, polyethylene terephthalates,        polytrimethylene terephthalates, polybutylene terephthalates,        polyamides, polycarbonates, polyvinylpyrrolidone,        polytetrafluoroethylene, polysulfones, polyethersulfones,        polyphenylene sulfides, polyetherimides, polyamide-imides,        polyetheretherketones, and polyaryletherketones, including        alloys and blends.    -   32. The method according to embodiment 28, wherein the fabric        comprises a multidirectional fabric, a unidirectional fabric or        a woven fabric.    -   33. The method according to embodiment 28, wherein the pultruded        sheet comprises 20-80% by volume carbon fibers, based on the        total volume of the fiber-reinforced resin composite.    -   34. The method according to embodiment 28, wherein the        fiber-reinforced resin composite is in a form of a pultruded        sheet comprising 20-80% by volume of carbon fibers fused with a        vinyl ester resin.    -   35. The method according to embodiment 28, wherein the        fiber-reinforced resin composite is in a form of a pultruded        sheet comprising 20-80% by volume of carbon fibers fused with an        epoxy resin.    -   36. The method according to any one of the embodiments 25-35,        wherein the composite panel is in the form of a spar cap.    -   37. An article comprising at least two components, where a        portion of a first component is adhesively bonded to a portion        of a second component by an electrically conductive adhesive        according to any one of the embodiments 1-9.    -   38. The article of claim 37 wherein the article is a spar cap        and wherein the two components forms a chamfer region.    -   39. The article according to embodiment 37, further comprising        the electrically conductive adhesive disposed over at least one        of the at least two components.    -   40. The article according to embodiment 38, and wherein the        electrically conductive adhesive extends beyond the chamfer        region onto both surfaces of the adjoining components.

The invention will be further explained by the following Examples, whichare intended to be purely exemplary of the invention, and should not beconsidered as limiting the invention in any way.

EXAMPLES Materials Used

Thermosetting epoxy resin available as Sikadur WTG-1280 (Part A) andcurative agent available as Sikadur WTG-1050 (Part B), were obtainedfrom Sika Corporation. ZOLTEK PX35 MF200 milled fibers (˜5 μm to 150 μmin length by ˜7 μm in diameter) were obtained from Zoltek Corporation.All materials were used as is.

Pultruded Profiles Used

Profiles of pultruded sheet used in the examples below:

-   -   62% Fiber Volume Fraction (FVF) carbon fiber with 38% Vinyl        ester    -   72% FVF carbon fiber with 28% Epoxy resin

Testing Methods

Volume (through-thickness) electrical conductivity can be determinedfrom electrical resistance per ANSI ANSI/ESD STM11.12, using a PROSTAT®PRS-801 Resistance System or similar instrument and PROSTAT PRF-911Concentric Ring or similar apparatus, as shown in FIG. 9 .

Basic Process Step 1 Method of Making an Electrically ConductiveAdhesive as Shown in FIGS. 1 and 2

Milled carbon fibers (PX35 MF200) were mixed into a thermosetting epoxyresin (Part A, Sikadur WTG-1280) of the two part adhesive. The milledcarbon fibers were added in various amounts in the range of 2-37.5weight %, based on the total weight of the adhesive composition. Themixing was done until the two components were thoroughly combined toform a uniform mixture. Then, the curative agent/hardener portion (PartB, Sikadur WTG-1050) was added to the Part A/milled fiber mixture in a50:50 ratio, according to the manufacturer's instructions, and mixeduntil thoroughly combined to form an electrically conductive adhesive.

As shown in FIGS. 1 and 2 , the step of mixing of the Part A/milledfiber mixture with Part B can be done in air or partially under vacuum.FIG. 3 shows the difference between when the mixing is done in airversus when the step of mixing of the Part A/milled fiber mixture withPart B was done under vacuum. As is clear from FIG. 3 , the mixing undervacuum provides certain advantages over when carried out in air, such asuniform mixing and no air bubbles. Air bubbles can be detrimental toattaining a thin uniform layer of adhesive over a panel.

Step 2: Method of Making a Composite Panel as Shown in FIGS. 4-6

A pultruded composite panel was formed by first applying the fully mixedelectrically conductive adhesive as a thin even layer on one side of oneof a first pultruded plates (62% Vf Vinyl ester or 72% Vf Epoxy resin),as shown in FIG. 4 . Then, the second pultruded plate is placed on topof the adhesive layer of the first pultruded plate to form a bondedstack of pultrude composite panel, as shown in FIG. 5 .

This process of applying a thin layer of electrically conductiveadhesive and adding subsequent plates to form a bonded stack ofpultruded composite panel can be continued for as many panels asdesired, as shown in FIG. 6 .

The bonded stack of pultruded composite panels was cured under vacuum atthe adhesive supplier's recommended cure cycle of curing under vacuum at70° C. for 5 hours.

Electrical conductivity testing was performed on composite panels toconfirm dramatic improvement in conductivity through the thickness ofthe stack, as shown in FIGS. 10-12 .

FIG. 10 shows a graph showing through-thickness electrical conductivityof a composite panel comprising two pultruded sheets adhesively bondedby the electrically conductive adhesive of the present invention made bytwo different mixing methods (regular mixing versus mixing done using aResodyn LabRam model ResonantAcoustic® Mixer (RAM)), as a function offiller content, wherein each pultruded sheet comprises 62% by volume ofcarbon fiber fused with a vinyl ester resin. As can be seen in FIG. 10 ,the electrical conductivity of composite panel formed usingResonantAcoustic® mixer had lower conductivity as compared to one donein air, even though the mixing under vacuum resulted in uniform mixturewith no air bubbles. This discrepancy was due to a portion of part Bbeing stuck on the walls of the LabRam mixer resulting in an amountratio of curative agent/hardener portion (Part B, Sikadur WTG-1050) tothe Part A/milled fiber mixture different from supplier suggested 50:50ratio.

FIG. 11 shows a graph showing through-thickness electrical conductivityof a composite panel comprising two pultruded sheets adhesively bondedby the electrically conductive adhesive of the present disclosure, as afunction of filler content, where each pultruded sheet comprises 62% byvolume of carbon fiber.

FIG. 12 shows a graph showing through-thickness electrical conductivityof a composite panel comprising two pultruded sheets adhesively bondedby the electrically conductive adhesive or the various embodiments ofthe present disclosure, made by two different mixing methods, as afunction of filler content, wherein each pultruded sheet comprises 72%by volume of carbon fiber fused with an epoxy resin.

FIGS. 11 and 12 shows that irrespective of the composition offiber-reinforced composite, the adhesive composition provided asurprising and dramatic increase in the electrical conductivity measuredthrough stack, as compared to neat adhesive and reaches close to that ofsingle panel (single profile).

FIG. 13 compares the effect of using the conductive adhesive to chamferregion of two plates. The first panel in the FIG. 13 shows the resultsof a non-conductive joint when struck by electrical energy. The energyarcs from both the nearby surface of one plate (a) to the nearby surfaceof the second plate (b) and through the non-conductive adhesive layeritself.

The second panel in the FIG. 13 shows the results of the same electricalstrike in a conductively bonded joint. In this panel, the electricityarcs using only one path only from nearby surface to nearby surface ofthe two plates. As shown in FIG. 13 , the conductive joint shows nodamage from the strike. A method of applying the conductive adhesive tothe surface area of each plate adjacent to the joint and the chamferwill reduce the potential for arcing near the joint.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the practice of the presentinvention without departing from the scope or spirit of the invention.Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention. It is intended that the specification and examples beconsidered as exemplary only, with a true scope and spirit of theinvention being indicated by the following claims.

1. An electrically conductive adhesive composition comprising: milledcarbon fibers dispersed in a thermosetting resin; and a curative agent.2. The electrically conductive adhesive composition according to claim1, wherein the thermosetting resin is an epoxide resin.
 3. Theelectrically conductive adhesive composition according to claim 1,wherein the curative agent comprises an amine functional group.
 4. Theelectrically conductive adhesive composition according to claim 1,wherein the thermosetting resin comprises at least one of epoxy resin,vinyl ester resins, phenolic resins, unsaturated polyesters, thermosetpolyurethanes, polyimides, bismaleimide resins, benzoxazine resins,silicone resins, and any combination, copolymer, and/or derivativethereof.
 5. The electrically conductive adhesive composition accordingto claim 1, wherein the milled carbon fibers are present in an amount inthe range of 1 to 20 weight %, based on the total weight of theelectrically conductive adhesive composition.
 6. The electricallyconductive adhesive composition according to claim 1, wherein the milledcarbon fibers have an average diameter in the range of 5 to 10 micronsand an average length in the range of 1 to 300 microns.
 7. Theelectrically conductive adhesive composition according to claim 1,wherein the adhesive composition has a linear conductivity in the rangeof 10⁻¹² to 10⁻⁸ Siemens/m (S/m).
 8. The electrically conductiveadhesive composition according to claim 1, wherein the milled carbonfibers are free of sizing.
 9. The electrically conductive adhesivecomposition according to claim 1, wherein the milled carbon fibers arepristine or recycled milled carbon fibers.
 10. The electricallyconductive adhesive composition according to claim 1, wherein the milledcarbon fibers are present in an amount in the range of 1 to 50 weight %,based on the total weight of the electrically conductive adhesivecomposition.
 11. The electrically conductive adhesive compositionaccording to claim 1, wherein the milled carbon fibers are present in anamount in the range of 21 to 50 weight %, based on the total weight ofthe electrically conductive adhesive composition.